Typically plants for burning granular materials include a preheater with associated precalciner, a kiln, e.g., a rotary kiln, and a cooler for cooling the burnt material by means of air subsequently utilized as combustion air.
Various constructions of such plants for burning and sintering of ores, lime and cement raw materials are known. Most frequently the preheater includes a string of cyclones through which the raw material passes in one direction from the inlet to the outlet in a heat exchange process with the exhaust gas from the rotary kiln passing in the opposite direction through the preheater to a dust precipitator, e.g., an electrostatic precipitator, positioned after the preheater and before a chimney.
Coolers for cooling the material burnt in the rotary kiln by means of cooling air are likewise well known, for instance in the form of a planetary cooler associated with the rotary kiln or of a stationary grate cooler into which the rotary kiln discharges the hot material.
The air utilized in the cooler is, through the heat exchange in the cooler, brought to a temperature of between 600.degree. and 900.degree. depending on the type of cooler, so that the heated cooling air has a high heat content, which it is advantageous to recover. The heated cooling air is therefore usually utilized partly as secondary combustion air in the burning process performed in the rotary kiln, and partly as combustion air in the precalciner to which it may be fed through a separate pipe directly from the cooler. Further, the heated spent cooling air together with exhaust gases from the kiln and/or the precalciner may be used in the preheater for preheating the treated material. Such plants are known, for instance, from British patent specification Nos. 1,433,109; 1,463,124 and 1,478,246.
Known single string preheater systems with associated precalciners work with two calcination stages in series, a precalcination stage and a postcalcination stage. Of these, one, often in the form of the lowermost riser pipe, is in combination with the lowermost preheater stage. Combustion air may be fed separately to each of the two calcination stages either as spent cooling air or in kiln exhaust gas. The use of two calcination stages is intended to ensure the best possible precalcination but includes the drawback of a more complicated construction and regulation system. Such systems are known, for instance, from British patent specification No. 1,406,965 and U.S. Pat. No. 4,183,762.
A single string preheater-precalciner of the hitherto known type does not therefore offer the same advantageous technical and economical working conditions as a multi-string preheater where the precalciner forms part of a separate calciner string and is fed entirely with spent cooling air, while the preheater string(s) are fed with kiln exhaust gas, and where an optimal control of the ratio of air drawn through the strings is obtained by fans in each string. Contrary hereto, the presence of the precalciner as an integral part of the single string preheater has made it difficult to avoid compromising the air-fuel balance both in the kiln and in the appertaining calciner due to the problems of regulating the two burning processes together and obtaining, at the same time, an optimal precalcination of the treated material.
I have invented an apparatus for burning granular or pulverulent materials in a kiln plant with a single string suspension preheater-precalciner which is not encumbered with the above disadvantages and which, in addition, offers the favorable working conditions of multi-string preheaters, but is considerably less expensive to construct, work, and maintain and far more space saving than the latter.